During the operation of a hydrocarbon well (i.e. a gas or oil well) various down-hole problems can arise including the deposition of scale which inhibits the hydrocarbon flow. In the North Sea, typical scale problems are related to the production of inorganic salts such as BaSO4, SrSO4, CaSO4 and CaCO3. These salts precipitate as scale which, if left untreated, causes scaling of subsurface and surface production equipment and/or tubing and, eventually, blockage of the well hole.
To prevent scale from forming in the system, a scale inhibitor is typically injected into the formation intermittently in so called “squeeze” treatments. In a “squeeze” treatment, a scale inhibitor, typically at concentrations between 5-20% by weight, is injected into the formation through a producer well hole after a pre-flush. After an over-flush and shut-in, well production is then resumed. Ideally the production water slowly leaches or washes out the retained scale inhibitor from the formation. More specifically the leaching process should ideally place a low, but still effective, concentration (e.g. around 1-100 ppm) of the scale inhibitor in the produced water to prevent scale deposition. Depending on the inhibitor retention and release properties in the formation, however, the effect of this treatment may last from one month to about 24 months. For economic reasons, a prolonged period of protection from scale formation is clearly desirable.
An ideal scale inhibitor return curve for scale inhibitor concentration is one where, after the over-flush is complete, the inhibitor desorbs into the produced water at a rate that provides a constant concentration that is the minimum required to prevent scale formation. Even more ideally, this process continues until all of the scale inhibitor squeezed into the formation is released in this way.
Typically, however, squeeze treatments do not provide ideal scale inhibitor return curves. Usually the concentration of scale inhibitor in the produced water is initially high, and much greater than that required to prevent scale formation, as a result of inhibitor failing to adsorb to the formation. Thereafter the concentration of scale inhibitor tends to decrease until it eventually falls below the minimum required to prevent scale deposition. The process is therefore inefficient as a large proportion of the inhibitor introduced in the squeeze treatment is returned almost immediately and does not serve to prevent scale deposition. Moreover regular repetition of scale inhibitor treatment is highly undesirable as oil production invariably needs to be stopped to allow the treatment to be carried out.
Various techniques have been used to try to increase the proportion of scale inhibitor that is retained in a well. For example, U.S. Pat. No. 5,181,567 and U.S. Pat. No. 5,038,861 each disclose a method of prolonging the useful life of scale inhibitors in oil wells that employs polyquaternary amines such as poly-(dimethylamine-co-epichlorohydrin) or poly-(diallyldimethylammonium chloride). In a typical process, an amine solution is injected into a well, followed by injection of a scale inhibitor. An over-flush of brine, also referred to as “a water flush”, is then applied. The well is then shut in for 20-24 hours before production recommences. There is no explanation provided as to how the polyquaternary amine functions to prolong the scale inhibitor life.
EP-A-2052050 also discloses a method of increasing the retention of a scale inhibitor within a hydrocarbon producing system wherein the system is contacted with a polymer formed from a diallyl ammonium salt and with a scale inhibitor. EP-A-2052050 teaches that the polymer formed from a diallyl ammonium salt and the scale inhibitor are preferably formulated in separate liquid carriers and that the polymer formed from the diallyl ammonium salt is preferably injected before the injection of the scale inhibitor. This is said to be advantageous as it allows the polymer to adsorb onto the rock before it interacts with scale inhibitor. It is described in EP-A-2052050 that the polymer and the scale inhibitor form a gel in situ. This gel is believed to act like a precipitate in that it is easily retained by the hydrocarbon producing system. The reaction to form the gel is, however, believed to be reversible therefore when the concentration of scale inhibitor in the production water decreases, some gel will dissolve thereby releasing scale inhibitor. One reason EP-A-2052050 advocates treating the formation with a polymer formed from a diallyl ammonium salt prior to injection of a scale inhibitor is to ensure that this gel forms at the formation surface. This physically prevents scale deposition from occurring and means that the chance of pore blockage occurring is reduced.
EP-A-2052050 mentions the possibility of combining the above-described treatment steps with pre-flush and/or after or over-flush steps. The field treatment test disclosed in EP-A-2052050, for example, discloses a squeeze treatment according to the following protocol:
Preflush: 10 m3 2% KCl (to prevent clay swelling)
Preflush 2: 60 m3 10% DADMAC homopolymer in 6% NaCl
Main pill: 220 m3 5% scale inhibitor in seawater
Over-flush: 200 m3 (0.1% inhibitor in seawater)
Shut in: 12 hours
It is taught in EP-A-2052050 that an over-flush is typically done following the addition of the polymer formed from a diallyl ammonium salt and scale inhibitor and that it serves to displace the polymer/scale inhibitor out of the well bore. In other words the purpose of the over-flush is simply to push the treatment solutions into the formation. The over-flush composition therefore typically comprises brine.
Despite the improvements in squeeze lifetime provided by the above-described methods, there is still a need for methods for further increasing the retention of scale inhibitors in oil wells, and in particular, for methods that further extend the effect of a squeeze treatment. In addition the methods should not damage the formation (e.g. significantly reduce permeability) and, due to increased environmental concerns, the chemicals employed should exhibit good biodegradation properties with low toxicity and low bioaccumulation.
It has now been found that the lifetime of scale inhibitor squeeze treatments can be improved by incorporating an ionic polymer such as a polyquaternary amine or polyamino acid into the over-flush, i.e. after the main treatment has been carried out. Advantageously such methods have been found to further extend the squeeze lifetime of those treatments disclosed in U.S. Pat. No. 5,181,567, U.S. Pat. No. 5,038,861 and EP-A-2052050 wherein a polymer for enhancing squeeze lifetime is included in the pre-flush and/or treatment step. The finding that the inclusion of an ionic polymer in the over-flush improves scale inhibitor lifetime is surprising since the conventional wisdom in the art is that the over-flush simply serves to push or displace the treatment composition into the formation.